Using Arduplane 3.9.5:
Normal hardware install (I2C spliter that is plugged into the I2C port of the Pixhawk 2.1, and the SDP3X airspeed sensor is plugged into the splitter).
Change ARSPD_BUS param from it’s default of 1 to 0. This changes the bus read from the external to the internal.
I don’t fully understand this but it works.
I also happen to have a MaxBiotic I2C sonar also plugged into the splitter and it work when the ARSPD_BUS param is set to either 1 or 0. By work I mean I can see it correctly measuring distance, I do not yet know if it helps to hold altitude as expected.